Frontier this writer's established in literature (5)
I believe the answer is:
limit
'frontier' is the definition.
(thesaurus)
'this writer's established in literature' is the wordplay.
'this writer's' becomes 'im' (the person writing the crossword clues).
'established in' means one lot of letters goes inside another.
'literature' becomes 'lit'.
'im' put inside 'lit' is 'LIMIT'.
(Other definitions for limit that I've seen before include "speed restriction applied" , "Frontier" , "Restrict - term" , "Constraint" , "Keep in" .)